648
are replaced by well-crystallized illite and chlorite, and
vitrinite reflectance is high, but temperatures are only 100
to 150 0 C (e.g., Krumm et al. 1988).
Mechanical diagenesis results from vertical and accompanying lateral stresses, caused by the overburden load of younger sediments, and possibly by additional stresses due to compressional tectonic movements. It expels pore water and leads to a rearrangement of the sediment particles and their fabric.
Chemical-mineralogical or, simply, chemical diagenesis includes dissolution and recrystallization of
primary sediment particles, as well as reprecipitation
of dissolved matter as cement in the pore spaces.
Dissolution and cementation rnay take place at different levels within the sediment and thus involve mass
transfer from one layer to another. Chemical
diagenesis is driven by the tendency of solid matter
to reach thermodynamic stability under the changing
conditions in the subsurface (increasing temperature
and pressure, possibly changing pore water chemistry). As a result, a limited number of stable minerals
are formed at the expense of various unstable phases.
In old and deeply buried shales, for example, the
illite-chlorite-quartz mineral assemblage is common,
regardless of the initial clay mineral association. Calcite and dolomite replace less stable skeletal carbonates, such as aragonite and high Mg clacite. Furthermore, all sorts of minerals having a large surface area
tend to dissolve and reprecipitate into more compact
particles and thus to achieve astate of lower energy.
All these processes promote the reduction of porosity and permeability, enhance the bulk density,
and affect the strength of the rock. The loss in porosity and thus in sediment thickness is referred to as
compaction. Compaction of a water-saturated sediment is always associated with the expulsion of pore
water; the escaping water generates the compaction
flow.
The processes and effects of both mechanical and
chemical diagenesis in various tectonic settings and
sedimentary sequences are very complex and can be
studied and described in several ways. In this text,
only a brief summary can be given, in which the aspects of compaction and subsurface flow are more
emphasized than detailed descriptions of the cement
minerals and their paragenesis. The latter aspects
have been frequently and extensively addressed in
many special articles and textbooks. Brief introductions to the diagenesis of carbonates, biogenie silica,
and evaporites are given in Sects. 3.4.7, 5.3.5, and
6.4.6.
The enormous body of publications on diagenesis includes,
e.g., Engelhardt (1973), Bathurst (1975), Magara (1978),
Larsen and Chilingar (1979), Parker and Sellwood (1983),
McDonald and Surdam (1984), Gautier (1986), Mumpton
(1986), Füchtbauer (1988), Pamell (1994), Monta nez et al.
(1997), Morad (1998).
Chapter 13 Diagenesis
Some references to more specific topics in this field are
listed below.
13.1.2 Various States of Sediment Consolidation
Subaqueous mechanical diagenesis starts irnmediately below the sediment-water interface and progressively alters all the mass physical properties of
the sediments versus depth. It controls or influences
the porosity, water content, bulk density, permeability and pore water flow, shear strength and rheologie
behavior, sensitivity to liquefaction and, finally, influences mass transfer via molecular diffusion and
advection. One has to distinguish between three
states of consolidation:
- Normal consolidation.
- Underconsolidation.
- Overconsolidation.
(1) In normally consolidated sediments, the fabric
strength or compaction state is in equilibrium with
the overburden pressure at all depths below the
sediment-water interface (Fig. 13.la). The overburden pressure can be expressed as total lithostatic
pressure (see below) or as effective vertical stress
(reduced by buoyancy) per unit area of a horizontal
plane within the sediment. In reality, such an effective vertical stress acts as a grain-to-grain stress at
the contact of individual particles. The state of compaction can be defined in terms of porosity, water
content, or bulk density. The pore water pressure is
equal to the hydrostatic pressure at the corresponding
depth; hence, if sediment accumulation has ended,
there is no upward or laterally directed pore water
flow. However, if a new increment of sediment is
added on top of such a sedimentary column (Llh in
Fig. 13.la), this equilibrium is disturbed. In order to
establish a new equilibrium between the increased
vertical stress and the compaction state, some pore
water must be expelled (see below). In a fairly permeable sediment, this pore water expulsion takes
place more or less simultaneously with the growth of
the sedimentary column, i.e., the sediment maintains
its state of normal consolidation. Normal consolidation can also be rnaintained in fine-grained, low-permeable sediments if the sedimentation rate is low and
the pore water has sufficient time to escape.
(2) Underconsolidation. If the sedimentation rate is
high and the growing sedimentary column has a low
permeability and becomes less and less permeable
with depth, then the pore water to be released by
prograding compaction cannot escape readily to the
sediment-water interface. Hence, compaction is delayed (Figs. 13.1b and 13.3c), and the sediment is in
astate of underconsolidation. In this state, the fabric
are replaced by well-crystallized illite and chlorite, and
vitrinite reflectance is high, but temperatures are only 100
to 150 0 C (e.g., Krumm et al. 1988).
Mechanical diagenesis results from vertical and accompanying lateral stresses, caused by the overburden load of younger sediments, and possibly by additional stresses due to compressional tectonic movements. It expels pore water and leads to a rearrangement of the sediment particles and their fabric.
Chemical-mineralogical or, simply, chemical diagenesis includes dissolution and recrystallization of
primary sediment particles, as well as reprecipitation
of dissolved matter as cement in the pore spaces.
Dissolution and cementation rnay take place at different levels within the sediment and thus involve mass
transfer from one layer to another. Chemical
diagenesis is driven by the tendency of solid matter
to reach thermodynamic stability under the changing
conditions in the subsurface (increasing temperature
and pressure, possibly changing pore water chemistry). As a result, a limited number of stable minerals
are formed at the expense of various unstable phases.
In old and deeply buried shales, for example, the
illite-chlorite-quartz mineral assemblage is common,
regardless of the initial clay mineral association. Calcite and dolomite replace less stable skeletal carbonates, such as aragonite and high Mg clacite. Furthermore, all sorts of minerals having a large surface area
tend to dissolve and reprecipitate into more compact
particles and thus to achieve astate of lower energy.
All these processes promote the reduction of porosity and permeability, enhance the bulk density,
and affect the strength of the rock. The loss in porosity and thus in sediment thickness is referred to as
compaction. Compaction of a water-saturated sediment is always associated with the expulsion of pore
water; the escaping water generates the compaction
flow.
The processes and effects of both mechanical and
chemical diagenesis in various tectonic settings and
sedimentary sequences are very complex and can be
studied and described in several ways. In this text,
only a brief summary can be given, in which the aspects of compaction and subsurface flow are more
emphasized than detailed descriptions of the cement
minerals and their paragenesis. The latter aspects
have been frequently and extensively addressed in
many special articles and textbooks. Brief introductions to the diagenesis of carbonates, biogenie silica,
and evaporites are given in Sects. 3.4.7, 5.3.5, and
6.4.6.
The enormous body of publications on diagenesis includes,
e.g., Engelhardt (1973), Bathurst (1975), Magara (1978),
Larsen and Chilingar (1979), Parker and Sellwood (1983),
McDonald and Surdam (1984), Gautier (1986), Mumpton
(1986), Füchtbauer (1988), Pamell (1994), Monta nez et al.
(1997), Morad (1998).
Chapter 13 Diagenesis
Some references to more specific topics in this field are
listed below.
13.1.2 Various States of Sediment Consolidation
Subaqueous mechanical diagenesis starts irnmediately below the sediment-water interface and progressively alters all the mass physical properties of
the sediments versus depth. It controls or influences
the porosity, water content, bulk density, permeability and pore water flow, shear strength and rheologie
behavior, sensitivity to liquefaction and, finally, influences mass transfer via molecular diffusion and
advection. One has to distinguish between three
states of consolidation:
- Normal consolidation.
- Underconsolidation.
- Overconsolidation.
(1) In normally consolidated sediments, the fabric
strength or compaction state is in equilibrium with
the overburden pressure at all depths below the
sediment-water interface (Fig. 13.la). The overburden pressure can be expressed as total lithostatic
pressure (see below) or as effective vertical stress
(reduced by buoyancy) per unit area of a horizontal
plane within the sediment. In reality, such an effective vertical stress acts as a grain-to-grain stress at
the contact of individual particles. The state of compaction can be defined in terms of porosity, water
content, or bulk density. The pore water pressure is
equal to the hydrostatic pressure at the corresponding
depth; hence, if sediment accumulation has ended,
there is no upward or laterally directed pore water
flow. However, if a new increment of sediment is
added on top of such a sedimentary column (Llh in
Fig. 13.la), this equilibrium is disturbed. In order to
establish a new equilibrium between the increased
vertical stress and the compaction state, some pore
water must be expelled (see below). In a fairly permeable sediment, this pore water expulsion takes
place more or less simultaneously with the growth of
the sedimentary column, i.e., the sediment maintains
its state of normal consolidation. Normal consolidation can also be rnaintained in fine-grained, low-permeable sediments if the sedimentation rate is low and
the pore water has sufficient time to escape.
(2) Underconsolidation. If the sedimentation rate is
high and the growing sedimentary column has a low
permeability and becomes less and less permeable
with depth, then the pore water to be released by
prograding compaction cannot escape readily to the
sediment-water interface. Hence, compaction is delayed (Figs. 13.1b and 13.3c), and the sediment is in
astate of underconsolidation. In this state, the fabric